Geographic Range

Hawaiian monk seals are endemic to the Hawaiian Archipelago and are found primarily in the tropical waters of the northwestern Hawaiian island chain. Reproductive sites include Kure atoll, Midway atoll, Pearl and Hermes Reef, Lisianski Island, and French frigate shoals. Small populations also occur on Necker and Nihoa and breeding populations have been found on the main Hawaiian islands, also known as the windward Hawaiian Islands (Baker and Johannos, 2004; Antonelis et al., 2003). (Antonelis, et al., 2003; Baker, 2004)

Habitat

Hawaiian monk seals utilize terrestrial and marine habitats. Sandy beaches are used for parturition, nursing, molting, and resting. The most common parturition sites are on sandy beaches with exposed protective reefs, which limit shark access and provide shelter from large surf (Atonelis et al., 2003). Hawaiian monk seals use protected waters to teach weaned pups to capture prey. They use vegetation along the beach perimeter to protect themselves from ultraviolet rays, wind, and rain, and may also use terrestrial vegetation as shelter while they sleep (Gilmartin, 1983). Hawaiian monk seals spend the majority of their time in the ocean resting, interacting with each other, and foraging. They spend much of their time at depths of about 100 m and less, although some have been observed foraging at depths of over 300 meters (Parish et al., 2002). (Antonelis, et al., 2003; Gilmartin, 1983; Parrish, et al., 2002)

Physical Description

Hawaiian monk seals exhibit sexual dimorphism, with females being larger than males. Females have an average length of 2.25 m and an average weight of 203 kg. Males have an average length of 2.1 m and an average weight of 169 kg. After birth, Hawaiian monk seals are covered in black lunago (fetal hair), which they molt upon weaning. As juveniles, they are silvery gray, with darker hair on their dorsal side and white hair on their ventral side (National Marine Fisheries Service, 2007). Adults have brown pelage that is slightly gray on their backs and gradually fades to yellow and then white near the ventral surface of the animal. They have broad, flat, and moderately small heads with large black eyes. Unlike other pinnipeds, their nostrils are located on top of their short snout. (National Marine Fisheries Service, 2007)

Reproduction

Copulation in Hawaiian monk seals is rarely observed but usually takes place about 1 to 5 km off shore. Although little is known about the reproductive success or the mating system used by these animals, sexual dimorphism in conjunction with open water mating suggests polygyny (Weckerly, 1998). As in other pinnipeds, males likely mount females by biting their backs and grasping them with their foreflippers (Antonelis et al., 2003). (Antonelis, et al., 2003; Weckerly, 1998)

Hawaiian monk seals are monestrous and give birth to a single pup each year during an extended birthing season, which begins in March, peaks in April, and ends in August (Boness et al., 1998). Females leave the ocean to give birth on beaches close to their most recent breeding site. Birthing takes place after an 11-month gestation period, which contains a 3-month period of delayed implantation. Females are solely responsible for rearing offspring, which takes about 40 days. While rearing their pups, females fast and may lose hundreds of pounds. Pups weigh between 14 and 17 kg at birth and between 50 and 100 kg at weaning (National Marine Fisheries Service, 2007). Females wean pups by suddenly abandoning them and returning to the ocean, after which they wait 3 to 4 weeks before mating again (Boness et al., 1998). Hawaiian monk seals are one of three pinniped species that commonly foster pups, the other two being grey seals (Halichoerus grypus) and northern elephant seals (Mirounga angustirostris; Boness et al., 1998). Both genders reach sexual maturity between the ages of 5 and 10 years old. Pups weaned at higher weights generally have a greater chance of survival and become sexually mature at an earlier age. (Boness, et al., 1998; National Marine Fisheries Service, 2007)

Females are the only sex that invest in pup development. Pup fostering, which only occurs in 2 other pinniped species (gray seals, Halichoerus grypus and northern elephant seals, Mirounga angustirostris), takes place when females rear their young in close proximity to one another and may occur when pups are separated from their mothers. Most females tolerate nursing attempts by orphaned pups. Although pup fostering appears to result in little to no reward for adopting mothers, fostered pups clearly benefit from their foster mother's care (Boness et al., 1998). (Boness, et al., 1998)

Behavior

Except during the breeding season, Hawaiian monk seals are generally solitary animals, living alone or in some cases in small groups. Males greatly outnumber females and can be extremely aggressive toward them, sometimes "mobbing" and killing them (Antonelis et al., 2003). They are nocturnal and are non-migratory. They spend much of their time at sea foraging, returning to shore to sleep. Generally, they stay close to their natal island, and only about 10% move to a different island during their lifetime (Gilmartin and Forcada, 2002). Hawaiian monk seals, along with other pinnipeds, lack the ability to rotate their hind flippers. As a result, they use their front flippers to move around while on land. They are extremely agile in the water and use their back flippers to steer their fusiform bodies. (Antonelis, et al., 2003; National Marine Fisheries Service, 2007)

Home Range

Range size varies according to the composition and density of local prey assemblages and the size and age structure of local monk seal colonies. However, about 95% of foraging areas are located within 38 km of the center of an individual's atoll or island. (Stewart, et al., 2006)

Communication and Perception

Food Habits

Hawaiian monk seals primarily prey upon teleost fishes, which make up roughly 80% of their diet. They appear to prefer fish belonging to the families Muranidae (marine eels), Labridae (wrasses), Holocentridae (squirrelfishes and soldierfishes), Balistidae (triggerfishes) and Scaridae (parrotfishes). Except for the beardfish family (Polymixiidae), which consists of deep sea benthic fishes, all fishes consumed by Hawaiian monk seals are shallow reef fishes (Goodman-Lowe, 1998). The remainder of the their diet consists of cephalopods and crustaceans, with the majority of consumed cephalopods being octopi. Hawaiian monk seals prey upon diurnal and nocturnal species of teleosts and cephalopods; however, juveniles tend to prey more heavily on nocturnal species (Goodman-Lowe, 1998). They primarily forage in shallow reefs (less than 100 m) near their natal atoll and foraging takes place near or at the sea floor. Individuals foraging in precious coral beds (Corallium rubrum) at depths of over 300 m have been recorded, where prey-capture rates may be higher (Parrish et al., 2002). (Goodman-Lowe, 1998; Parrish, et al., 2002)

Predation

The main predator of Hawaiian monk seals is the tiger shark (Galeocerdo cuvier), and shark predation is believed to be a significant contributing factor to pup mortality. Although male Hawaiian monk seals are known to commit infanticide, sharks are thought to be the main threat to pup survival. Seals avoid potential predators by nesting on beaches protected by exposed reefs and by feeding and resting in underwater reef caves. (Gilmartin, 1983)

Ecosystem Roles

Hawaiian monk seals prey upon 40 species of marine animals that live in the coral reef ecosystem, including many species of teleost fish, cephalopods, and crustaceans. (Gilmartin and Forcada, 2002)

Economic Importance for Humans: Positive

Currently, Hawaiian monk seals have little to no economic importance. However, during the mid-19th century, when Hawaii was first claimed by the United States, Hawaiian monk seals were killed for their skin, oil, and meat. (National Marine Fisheries Service, 2007)

Economic Importance for Humans: Negative

Present day Hawaiian monk seal populations represent a fraction of their historical abundances. Although recovery efforts are now underway, the total cost of these efforts over the next 50 years is estimated to be about 385 million dollars. Also, in compliance with the National Marine Fisheries Service recovery plan for Hawaiian monk seals, important feeding and reproductive sites, typically beaches, have been closed to public access. Finally, they occasionally remove fish from commercial trolling lines. (Antonelis, et al., 2003; National Marine Fisheries Service, 2007)

Conservation Status

Hawaiian monk seals have been on the United States Fish and Wildlife Service’s endangered species list since November 23, 1976 and are listed as critically endangered on the IUCN Red List of Threatened Species. They have been in decline for over 20 years, and as of 2007, only 1200 individuals remained in the wild. Experts estimate that fewer than 1000 individuals will remain in the wild by the end of 2012. Efforts by the National Marine Fisheries Service to stabilize population numbers include keeping tourists away from known reproductive sites, moving aggressive males to new breeding grounds, and implementing a captive care program, which provides females with nutritional supplements. The goal of the captive care program is to increase the survival rate of female juvenile seals, which have an extremely low survival rate. Hawaiian monk seals are vulnerable to introduced disease, inbreeding depression, low genetic diversity, human disturbance, and competition with fisheries. In addition to the U.S. Fish and Wildlife Service’s Endangered Species Act, they are also protected under the Marine Mammal Protection Act. (Antonelis, et al., 2003; National Marine Fisheries Service, 2007)

Hawaiian monk seals are intolerant of human disturbance on beaches used for birthing and resting, and pup mortality is often higher at disturbed sites. Hawaiian monk seals began declining in the mid to late 1800’s, when they were hunted for their meat and skins. Currently, populations are declining due to over fishing and seals becoming hooked or entangled in fishing gear (Antonelis et al., 2003). (Antonelis, et al., 2003; National Marine Fisheries Service, 2007)

Glossary

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

delayed implantation

in mammals, a condition in which a fertilized egg reaches the uterus but delays its implantation in the uterine lining, sometimes for several months.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

island endemic

animals that live only on an island or set of islands.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

molluscivore

eats mollusks, members of Phylum Mollusca

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

piscivore

an animal that mainly eats fish

polygynous

having more than one female as a mate at one time

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

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The Animal Diversity Web is an educational resource written largely by and for college students. ADW doesn't cover all species in the world, nor does it include all the latest scientific information about organisms we describe. Though we edit our accounts for accuracy, we cannot guarantee all information in those accounts. While ADW staff and contributors provide references to books and websites that we believe are reputable, we cannot necessarily endorse the contents of references beyond our control.

This material is based upon work supported by the
National Science Foundation
Grants DRL 0089283, DRL 0628151, DUE 0633095, DRL 0918590, and DUE 1122742. Additional support has come from the Marisla Foundation, UM College of Literature, Science, and the Arts, Museum of Zoology, and Information and Technology Services.